The millions of cars and trucks on the road throughout the world represent a substantial source of air pollution. To minimize pollution, many countries have enacted clean air laws restricting the amount of pollution that vehicles can produce. One method employed by auto manufacturers to reduce such pollution is the use of a catalytic converter which treats the exhaust gases of vehicles to reduce some pollutants. Theoretically, vehicles are designed with an air-to-fuel ratio such that all of the fuel is burned using all of the oxygen in the air within the combustion chamber. However, the ideal air-to-fuel mixture varies during driving. The main emissions of a typical car engine include nitrogen gas, carbon dioxide, and water vapor. The emissions just mentioned are relatively benign. However, the combustion process is never perfect and small amounts of more harmful emissions are also produced. These harmful emissions are a part of the six main pollutants the EPA has identified as “Criteria Pollutants.”
The six Criteria Pollutants include: (1) ozone; (2) volatile organic compounds (VOCs); (3) nitrogen dioxide (NO2); (4) carbon monoxide (CO); (5) particulate matter (PM); and (6) sulfur dioxide (SO2). Ozone is created by the chemical reaction of pollutants and includes VOCs and NOx. In addition, ground-level ozone is the principle component of smog. VOCs (volatile organic compounds) are released from burning fuels (gasoline, oil, wood, coal, natural gas, etc.), solvents, paints glues and other products used at work or at home. Automobiles are large contributors to the amount of VOCs pollutants. VOCs also include chemicals such as benzene, toluene, methylene chloride and methyl chloroform. Nitrogen Dioxide (NO2) is one of the NOx pollutants and is a smog-forming chemical. NO2 is created by the burning of gasoline, natural gas, coal, oil, and the like. Automobiles are large contributors to the amount of NO2 pollutants. Carbon Monoxide (CO) is created by the burning of gasoline, natural gas, coal, oil and the like and automobiles are large contributors to the amount of CO pollutants. Particulate Matter (PM)-10 can be dust, smoke, and soot and can be created by the burning of wood, diesel, and other fuels. Industrial plants, agricultural activities such as plowing and burning off fields, and the use of unpaved roads contribute to the amount of PM pollutants. Finally, sulfur dioxide (SO2) is created by the burning of oil, especially high-sulfur coal from the Eastern United States, and through industrial processes (paper, metals).
Often catalytic converters will use catalysts to enhance or aide in the filtering of engine exhausts to reduce the amount of Criteria Pollutants. Typical catalytic converters use two different types of catalysts, a reduction catalyst and an oxidation catalyst. Most catalyst filters consist of a ceramic structure coated or impregnated with a metal catalyst such as platinum, rhodium or paladium. The idea behind a catalyst exhaust filter is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream while minimizing the amount of catalyst required due to the high cost of such catalysts.
As seen in FIG. 1, a prior art catalytic converter 100 includes a reduction catalyst 102 and an oxidation catalyst 104. As exhaust enters the catalytic converter 100 it is filtered and exposed to the reduction catalyst 102. The reduction catalyst 102 typically uses platinum or rhodium to help convert the nitrogen oxides within the emissions to less harmful substances. The nitrogen oxide molecules contact the catalyst which momentarily retains the nitrogen atom freeing the oxygen in the form of O2. The nitrogen atom binds with other nitrogen atoms stuck to the catalyst forming N2.
The exhaust is then treated by the oxidation catalyst 104 which causes unburned hydro-carbons and carbon monoxide to burn further. (The oxidation catalyst aids the reaction of the carbon monoxide (CO) and hydro-carbons with the remaining oxygen in the exhaust gas.) The primary structure of converters is a porous honeycomb having small tubules. FIG. 2 shows an example of a ceramic exhaust filter incorporating a honeycomb catalyst structure.
Diesel engines (where compression alone ignites the fuel) have recently come under worldwide scrutiny for their exhaust emissions which contain a larger number of harmful particulates in addition to toxic gases. Manufacturers' response has been to apply known catalytic converter technology to diesel engines apparently assuming that one solution will work for all types of fossil fuel pollution. Unfortunately, regulations regarding emission standards have exceeded the physical and economic limits of conventional catalytic converter technology. Diesel emissions are different than gasoline emissions, especially in the greater amount of particulate matter generated. For these reasons, existing technology for exhaust emission capture, combustion, and oxidation will not comply with the increased diesel engine emission standards required.
Commercial solutions which have been developed to meet these new diesel engine emission standards can be categorized into two viable groups: (1) conventional monolithic catalytic converters with a honeycomb configuration; and (2) inorganic fiber cartridges. It is commonly known that particulate matter, in the form of exhaust emission of unburned hydrocarbons, needs to be captured and completely combusted or burned. This capture is accomplished by placing a porous septum in the path of the exiting emission which allows the particulate matter to bond or adhere to the septum through surface tension. The porous septum also permits the accompanying gases to pass through the pores as unrestricted as possible. The septum is likened to a spider web laid out to capture flying insects.
Once the particulate matter is captured, the particulate needs to be completely combusted or burned by raising the particulates temperature in an oxidizing environment. Combustion of the particulates can be accomplished by utilizing the existing temperature of the exiting exhaust and/or providing an auxiliary source of heat. A known problem is that the temperature required to accomplish combustion must also have the particulate matter reside on the septum surface for a length of time. This period of time is called residence time.
FIG. 2 provides a graph of the residence time required to combust or burn particulate matter (soot mass) at various temperatures. As seen in FIG. 2, the residence time to combust or burn soot mass having a 0.9 soot mass at 600 degrees (Kelvin) is much longer than the residence time at 1200 degrees. The longer the residence time, the smaller the allowable through put volumes and the greater the risk of more particulate accumulating on and clogging the septum pores. Clogging can also be a result of the ceramic material overheating to the point of melting thereby blocking or clogging the septum pores. In order to prevent clogging, obstruction or saturation more surface area is required. A useable solution must consider which temperature: (1) provides the lowest residence time; (2) is safest from thermal harm; (3) uses a minimal amount of auxiliary energy; and (4) is inexpensive to produce. Increasing temperature requires additional energy. Further, certain amounts of the energy source are conducted, drawn, or channeled away by coming in contact with a material through thermal conductivity. The chemistry of different substances determines the level of thermal conductivity. Additionally, the thermal conductivity of the filter medium determines the efficiency of the exhaust emission filter. A low thermal conductivity is preferred because more of the heat energy generated is reflected back, and will remain in the pore space if the solid portion of the filter does not conduct or channel heat away. The lower the thermal conductivity, the lower the loss of heat. Lower heat loss translates into less energy needed to obtain the desired temperature range for catalytic conversion and higher energy efficiency.
Since all materials have some level of thermal conductivity, it is preferable to minimize this effect. Conductivity minimization can be accomplished by choosing a material with a lower conductivity or by using less of the material.
As previously discussed in conjunction with FIG. 2, a higher temperature permits the particulate matter to combust with a shorter residence time and therefore, increased heat is preferred. Moving the filter closer to the combustion chamber or engine or adding an auxiliary heat source can provide increased heat. However, conventional catalytic converter filter elements cannot withstand the high temperatures and increased vibrational shock present in such locations. In addition, some catalysts applied to conventional filter elements will work less efficiently or even cease to function at high temperatures (i.e. above 500° C.). Therefore, what is needed is a filter element which can be placed in extremely high temperatures (i.e. above 500° C.), such as near the combustion chamber, is more resistant to vibration degradation, and still has the same or an increased particulate matter burning effect. The ability to achieve the same particulate matter burning effect without a catalyst will also provide significant savings on catalyst and coating costs.
Further, the addition of an oxidation catalyst coating applied to the filter can provide the same combustion and oxidation effect at a lower and more reasonable temperature. As previously mentioned, metal oxidation catalysts such as platinum, palladium, or rhodium are preferred. The end result is that catalytic coatings lower the hydrocarbon combustion temperature range allowing a more flexible and reasonable distance between the filter and the engine.
The features needed for providing an improved exhaust emission system includes filter with a minimum a mass and maximum surface area. Additional features which directly influence and determine the primary features are thermal conductivity, thermal expansion, thermal shock, vibrational shock, stress tolerance, porosity, permeability, weight, cost to produce, ease of manufacture, durability, as well as others.
As seen in FIG. 3, in order to increase the surface area for these catalytic converters a honeycomb configuration 302 or structure is formed within the ceramic filter element 300. The honeycomb structure 302 is formed using an extrusion process in which long tubes with their major axis parallel to the extrusion action are created. The opening of these tubes faces the incoming exhaust airflow. As the emissions enter the tube the particulate will deposit along the interior septum of the tubes. The honeycomb configuration 302 substantially increases the surface area permitting more particulate to be deposited in less volume.
In the internal combustion emission-filtering market the automobile or gasoline engine catalytic converter is the dominant technology. Existing catalytic converter technology is primarily based on a high temperature ceramic, such as cordierite (2 MgO-2Al2O3-5SiO2) or silicon carbide (SiC). These ceramics are usually extruded into a honeycomb pattern from slurry and then heat-cured into the rigid form of the extrusion. There are physical limits to either cordierite or silicon carbide. Additionally, continued refining of the extrusion process to produce a thinner septum, from 0.6-1.0 mm to 0.2-0.4 mm, has reduced the mass. After over thirty years of refinement, the extrusion process has achieved near physical limits for economic catalytic applications.
Cordierite has been used throughout most of the automobile industry's catalytic converter history and it worked well during the early phase of automotive pollution control. However, with new and stricter regulations enacted worldwide, cordierite in its current configuration cannot provide sufficient emission control. The honeycomb septums are as thin as can be economically extruded. Chemically, the ceramic density has been reduced from 60% plus to the low 40 percentile. In order for these filters to accommodate the increased volume of particulate generated by a diesel engine, the filter sizes have to increase, which adds to vehicle weight, manufacturing costs and operating costs. The percentage of particulate captured with cordierite filters is around 73%, but it continually declines over time due to clogging. At the beginning of the filter's life, the ceramic is 100% clean but the remaining 27% of particulate not captured will build up on the septum walls and the filter will eventually fail to operate. Failure of the filter takes approximately 100,000 miles which coincides with the manufacturer's; recommended filter replacement schedule.
In some instances, cordierite is being replaced by silicon carbide since it has superior heat resistance. Compression ignition engine exhaust temperatures can be greater than that of spark ignition and thus the higher operating temperatures make silicon carbide preferable to cordierite for diesel engines. Cordierite begins to decompose at approximately 1,400 degrees C. while silicon carbide can withstand temperatures up to approximately 2,000 degrees C. However, silicon carbide has a greater thermal expansion and is more costly. Silicon carbide is also much heavier than cordierite and any additional weight is detrimental to vehicle performance. Silicon carbide catalytic converters can be chemically modified to increase porosity through the addition of inorganic fibers. The end result is a minor improvement in particulate filtering of approximately 80%, which translates into a filter life of about 120,000 miles before requiring filter replacement.
Both cordierite and silicon carbide filters have a poor resistance to vibrational and thermal shock. As such, these filters cannot be placed immediately next to or inside an engine exhaust manifold, which is the best location to take advantage of the in situ high temperatures before the temperature decreases due to radiant cooling from the high thermal conducting properties of the exhaust pipe material. Engine vibration and the quick change in temperatures that exist near and within the exhaust manifold would cause the filter material to fatigue and dramatically shorten the life of the filters resulting in filter failure.
The extrusion process used to create the filters also restricts the filter shapes used to near cylindrical bodies formed along the major axis of the extrusion. The shape limitation has not been an issue with previous emission standards. However, the need to design filters to reach near-zero emissions performance may require non-linear and/or non-cylindrical filter design and vehicle integration.
The inorganic fiber cartridges evolved from fossil fuel energy plant filter systems. Energy plants, in particular coal-burning plants, generate large quantities of particulate matter. Particulates are removed by passing the emissions through a series of tubes sealed at one end and wrapped in layers of inorganic fiber. These wrapped tubes are referred to as cartridges. In some instances, the wrapped tubes are referred to as candle filters because of their visual similarity to candles. These are effective when they are in a stationary, open environment with no requirement for small space configuration, and safety from the heat is a minor requirement.
The basic functionality of the cartridge is to direct the exhaust emissions into a series of tubes with one end blocked off. Each tube is perforated and the tube exterior is covered with layers of inorganic fibers. The inorganic fibers are secured to the outside of the tube by wrapping yarn or fabric forms of the fiber around the tube. The wound fiber material is secured and made rigid with an inorganic binder and then heat cured.
Several of the cartridges are placed in a cluster with their major axis' parallel to each other. The major axis of the stack is placed perpendicular to the exhaust emission gas flow forcing the gases to enter into and pass through the inside of the tube and exit then through the fiber covering as exhaust. Scaling down a large candle filter into a vehicle exhaust cartridge configuration offers considerable challenges. First, the creation of these filter cartridges is very labor intensive, expensive to build, and to install. Second, the intolerance to vibrational shock in a mobile environment can produce fatigue over time from all of the various interactions of parts, such as plates, tubes, screws, and mounting brackets for each cartridge. Additionally, the interaction of the cartridges against each other in the filter assembly produces fatigue and failure. Third, the end product would still remain relatively large and has definite limitations to scaling down. Fourth, the surface area is essentially equal to or less than traditional catalytic converters. Fifth, the weight is heavy from all of the different parts. Finally, the amount of particulates trapped and combusted and the residence time required does not provide significant improvement in filtration and performance. Overall, a system which uses inorganic fiber cartridges for engine exhaust filtering is too convoluted and complicated to be economically successful in automobiles. However, the use of inorganic fibers does have positive properties. For example, the thermal expansion and the heat conductance of the fibers are very low. In addition, the amount of mass used in the combustion process is good.
Therefore, what is needed is an improved exhaust filter which provides an economic and porous substance which can be shaped or formed to provide a large amount of surface area, with a low thermal expansion and heat conductance, in a filter which can withstand high levels of heat and vibration.